Physics on the Boundary between Classical and Quantum Mechanics

Quantum Mechanics and Classical mechanics are usually regarded as mutually exclusive: a system is presumed to be either quantum mechanical or classical but never both. Here, however, we show that theories that are both do exist — there is life on the boundary. These theories allow for dual mappings between a classical picture and a quantum one; they both describe the same time evolution equally well. There are some simple examples, but also quite contrived ones, such as superstring theory.